SQL - Foreign Key - In SQL, a Foreign Key is a column in one table that matches a Primary Key in another table, allowing the two tables to be connected together.
SQL定义功能中,用CREATE TABLE建立表时,FOREIGN KEY...REFERENCES…短语的含义是A.说明主关键字B.建立表之间的联系C.说明有效性规则
FOREIGN KEY (student_id) REFERENCES student(id) ON UPDATE CASCADE ON DELETE SET NULL ); 在上面的例子中,当更新主表的id列时,从表中与之相关的记录将会被级联更新。当删除主表的记录时,与之相关的记录的外键列将会被设为NULL。 总结: 在SQL中使用FOREIGN KEY约束能够建立表与表之间的关系,并确保数据的...
Again, it means that there exists a one-to-many relationship between the two tables, but this time one employee can work atonly onecompany, which is in line with the rules for the scenario. Add a Foreign Key to Enforce Referential Integrity Between Two Tables Even though we have created t...
短语的含义是 A.说明主关键字B.建立表之间的联系C.说明有效性规则D.说明根据指定数组的内容建立表 相关知识点: 试题来源: 解析 B [解析] 在使用SQL语句CREATE TABLE建立表时,可使用印FOREIGN KEY…REFERENCES…短语来建立表之间的联系。反馈 收藏
The structure of these two tables will be as follows: TableCUSTOMER Column NameCharacteristic SIDPrimary Key Last_Name First_Name TableORDERS Column NameCharacteristic Order_IDPrimary Key Order_Date Customer_SIDForeign Key Amount In the above example, the Customer_SID column in theORDERStable is a...
TheFOREIGN KEYconstraint in SQL establishes a relationship between two tables by linking columns in one table to those in another. For example, Example: Foreign Key in SQL Here, thecustomer_idfield in theOrderstable is aFOREIGN KEYthat references thecustomer_idfield in theCustomerstable. ...
FOREIGNKEY(PersonID)REFERENCESPersons(PersonID); DROP a FOREIGN KEY Constraint To drop aFOREIGN KEYconstraint, use the following SQL: MySQL: ALTERTABLEOrders DROPFOREIGNKEYFK_PersonOrder; SQL Server / Oracle / MS Access: ALTERTABLEOrders DROPCONSTRAINTFK_PersonOrder; ...
Here you will learn what is a foreign key and how to established a relationship between two tables using a foreign key in the SQL Server database. What is Foreign Key? The foreign key establishes the relationship between the two tables and enforces referential integrity in the SQL Server. Fo...
SQL FOREIGN KEY ConstraintA FOREIGN KEY in one table points to a PRIMARY KEY in another table.Let's illustrate the foreign key with an example. Look at the following two tables:The "Persons" table:P_IdLastNameFirstNameAddressCity 1 Hansen Ola Timoteivn 10 Sandnes 2 Svendson Tove Borgvn ...